1. Field of the Invention
The present invention relates primarily to digital magnetic recording systems. The invention may be considered as a system that converts binary data into ternary data and records the ternary data on a magnetic medium. The ternary data read from the medium is converted back into the corresponding binary format. The concepts of the present invention may also be utilized in digital communication systems. Other ternary systems are disclosed in the applications of Chao S. Chi for "Controlled Return to A.C. Digital Magnetic Recording and Reproducing System", Ser. No. 339,352, filed concurrently herewith, and in the application of George Jacoby and Martin Cohen for "Ternary Data Encoding System", Ser. No. 260,248, filed May 5, 1981.
2. Description of the Prior Art
Present day digital magnetic recording systems typically record binary data by utilizing binary valued write current waveforms. For example, in the conventional return-to-zero (RZ) and non-return-to-zero (NRZ) formats each cell of the medium is magnetized either in the positive direction or the negative direction to represent the two binary data states. In a modified NRZ format (NRZI) a transition from the existing polarity of magnetization to the opposite polarity of magnetization is recorded for a binary ONE and no transition is recorded for a binary ZERO.
It is a desideratum of the magnetic recording art to increase the information density stored on the medium by, inter alia, packing the magnetic flux transitions as closely as possible on the medium. Depending on the configuration of the magnetic interface, non-linear distortions and intersymbol interference degrade data recovery reliability because of such factors as pattern dependent amplitude attenuation, which affects bit resolution, and timing displacement anomalies such as peak shift. Conventionally complex and hence expensive signal processing channels are provided to enhance data recovery reliability in such systems utilizing high bit packing density of the medium.
Another technique utilized to increase the storage efficiency of the medium is to encode the binary data by various run-length-limited codes. Although providing significant improvements in recording density efficiency, systems utilizing such codes are nevertheless restricted by the basic limitations of the interface.
It should be noted that only the position of the transition, not the polarity of the transition, has been commonly used to convey information in the present day saturated digital magnetic recording systems. Such a system may be considered as having only one degree of freedom and is limited to binary data.
A shortcoming of such prior art systems is manifested in the inability to freely choose between different transition polarities while maintaining the medium in saturation.
Another basic requirement in digital magnetic recording systems is overwrite of old data. When new data is recorded in prior art saturated magnetic recording systems, saturation current must be utilized to effectively erase the old data. Thus, a recording system utilizing plus and minus flux saturation in the traditional manner had the advantage of always saturating the medium so that recording new data automatically erased the old. However, since only two saturated states were possible the system was limited to binary data.
It is generally known that the information density of magnetic recording systems may be enhanced by providing higher order recording than binary. For example, the NRZ and RZ formats may be enhanced by utilizing the zero write current state as an information bearing condition in conjunction with positive and negative magnetization. Such a ternary system will not provide the necessary function of overwrite of old data when new data is recorded. A separate erase cycle may be utilized to obviate the problem but this is generally considered unacceptable in high speed present day digital magnetic recording systems.
The problem of enhancing information density by providing higher orders states is a problem of selecting signal waveforms having multiple degrees of freedom (to represent higher order states) which are not distorted beyond recognition by the magnetic recording system. This is a problem of considerable complexity due to severe waveform distortion and intersymbol interference that can occur at higher densities, and due to amplitude instability and dropouts caused by imperfections in the magnetic media. The problem is further complicated by the fact that the inductive readback magnetic recording system has a limited bandpass transfer function, incapable of reproducing very long or very short wavelengths, and by the fact that the magnetic medium is non-linear unless kept constantly saturated or otherwise impressed with an a.c. bias.